1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a sheet.
2. Description of the Related Art
In the related art, an image forming apparatus, such as a copying machine, a laser beam printer, and a facsimile, transfers a toner image formed on an image bearing member onto a sheet and then heats the sheet by a fixing device to fix the toner image.
In recent years, the image quality of the image forming apparatus including a sheet conveying apparatus which conveys a sheet has been enhanced. The image forming apparatus flexibly copes with a thick coat sheet having a large thermal capacity. When such thick coat sheet having a large thermal capacity is used, the sheet which becomes very hot by being heated by the fixing device before it is conveyed to a discharge portion.
When the sheet is conveyed while still hot, toner is softened and is not fixed onto the sheet. The sheet is brought into contact with a conveying roller or the rib of a conveying guide arranged in a conveying path while still hot. As a result, there can be a roller trace or a rib trace on the toner image to cause unevenness on the image. When the hot sheet is discharged and stacked onto a discharge tray, the soft toner acts as an adhesive and can adhere onto the sheet which has been already discharged onto the discharge tray.
To solve such problems, before the sheet is discharged onto the discharge tray, the sheet is cooled by blowing air thereonto to harden the sheet (Japanese Patent Application Laid-Open No. 2001-255807).
Japanese Patent Application Laid-Open No. 2005-112568 and Japanese Patent Application Publication No. 4-68629 disclose an apparatus having an opening which passes air sent from a fan therethrough and is formed in a switching member which switches between two branched conveying paths to guide a sheet. In the configuration of Japanese Patent Application Laid-Open No. 2005-112568 and Japanese Patent Application Publication No. 4-68629, air flows into the branched conveying path branched from a conveying path through the opening of the switching member. The branched conveying path is a conveying path which guides the sheet to be switchback conveyed. The branched conveying path into which air flows is a conveying path which guides the sheet before being switchback conveyed and the sheet after being switchback conveyed. The branched conveying path is shared so as not to be branched into the conveying path which conveys the sheet before being switchback conveyed and the conveying path which guides the sheet after being switchback conveyed. The conveying interval between the previous sheet and the following sheet need to be long. The productivity of sheet conveying is limited.
As illustrated in FIG. 12, a fan F4 is provided in a forward conveying path R1 which conveys the sheet with a toner image fixed onto one side thereof by a fixing portion 200 to a discharge roller 201. In addition, a fan F5 which blows outside air onto the sheet with the toner image fixed onto one side thereof by the fixing portion 200 is provided in a reverse conveying path R2 which reverses the sheet and then conveys it to the discharge roller 201.
When the fans F4 and F5 are provided in the forward conveying path R1 and the reverse conveying path R2, respectively, the cost and the size of the apparatus can be increased. Exhaust can be delayed due to the influence of the outside air blown from the fans F4 and F5 in two directions to increase the temperature in the apparatus body.
In No. 2006/0133865 (U.S. Patent Publication), to cool the sheets in different conveying paths by one fan, there is provided an airflow changing member which opens and closes a plurality of blowports provided corresponding to the conveying paths, respectively, to switch between the blowports which blow out air. There is provided the dedicated airflow changing member which switches between the conveying paths to be cooled. The size of the apparatus and the cost can be increased.